Many methods have been utilized to measure the AC current flowing to a load. These methods are used as part of more complex devices such as solid state breakers or overload relays, AC line monitoring devices, or current feedback control loops as required for motor drives and power supplies. The application usually determines the degree of measurement accuracy required. Root-mean-square (rms) current is generally considered to be an accurate representation of the actual current flowing in the circuit being monitored. For simple systems, this may not be necessary and instead, average current over a set time period, usually one cycle, is used to represent the actual current. Other systems will use peak current over the same time period as the basis. More sophisticated microprocessor based devices will actually compute the rms value by sampling the current waveforms many times during each time period, and using those sample measurements to generate the value of the current over the time period.
Using a solid state overload relay as an example of one such application, a common method used to generate a signal representative of the line current is to use a peak detecting circuit as disclosed in commonly assigned U.S. Pat. No. 4,345,288. Although this device provides a close approximation of a standard thermal electromechanical overload unit, the device does not compensate for non-sinusoidal currents that may be encountered in some instances. Likewise, line harmonics and transients may cause erroneous results. A microprocessor based device to accomplish a similar function is outlined in U.S. Pat. No. 5,220,478 which will compute the rms current through digital sampling techniques. Whereas this is a more accurate method, it is a relatively expensive solution because of the increased component requirements for support circuitry and the increase in the size of the device to house the extra components. For a low cost application where size is also a factor, it would be desirable to provide a solid state overload relay that computes an approximation of the rms current without the complexity of a microprocessor based device.